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a) Fresh apricot purée.
After washing, cutting and removal of stones, apricot halves are dipped in 2% solution of sodium or potassium metabisulphite for 10 minutes. After draining, the resulting material is passed through a 0.045-in. screen pulper - finisher to produce a fresh apricot puree.
The fresh apricot puree obtained in this way could be further processed in different semiprocessed (i.e. chemically or otherwise preserved products) or finished fruit products (fruit leathers, fruit bars, jams, etc.).
b) Concentrated apricot pulp.
Fresh apricot halves could also be steam blanched for 5 min., passed through a 0.045-in pulper - finisher and transformed in a purée with about 14 Brix depending on the fruit quality. This purée may be concentrated in steam jacketed kettles up to 20 Brix or in other adequate equipment (e.g. a stirred vacuum evaporator) up to 28°Brix.
As for fresh apricot purée, the concentrate may be further processed in various semiprocessed or finished fruit products as mentioned above and as will be described below.
c. Dried apricot leather.
a) Trays. - For sun / solar drying or dehydration of fruit pulp, the trays must have a solid base in order to retain the liquid contents. They may be made of metal, timber or plastic. Stainless steel or plastic trays are most suitable because they are unaffected by acid fruit pulp; they are, however, expensive.
A metal tray could be 75 x 50 cm in size and with side 5 cm high. The trays must keep level during drying; if the tray is not level the pulp will run to the lowest point, giving a layer of irregular depth which will dry unevenly. Any tray which is not made of stainless steel or plastic must be covered inside with a sheet of heavy gauge plastic film to protect the pulp from chemical or bacteriological contamination.
Standard sun/solar trays as described can be used by covering them inside with a sheet of plastic film to create a solid base.
b) Preparation before drying/dehydration. Fresh apricot purée can be directly used for next processing steps. Fruit concentrate needs to be added to potassium metabisulphite to obtain a 0.3% concentration of SO2 in the material.
c) Drying/dehydration. The apricot/fruit purée or concentrate is poured into the trays to a depth of about 1.5 cm. When stainless steel or plastic trays are used they should be coated with a thin layer of glycerine to prevent sticking.
The pulp is then sun/solar dried or tunnel/cabinet dehydrated; moisture content in the dried product should not exceed 14% and the SO2 content should not be less than 1500 pp.
The dried product is wrapped in cellophane to prevent sticking, then put inside polythene bags and stored at best in tight fitting tins and sealed to prevent moisture transfer.
- From fresh fruit purée or from apricot concentrate, with sugar addition, and then processed by sun / solar drying or by dehydration.
In some countries preference is for finished products with added sugar; and this is also interesting from a point of view of energy consumption (concentration is partially achieved by sugar dry matter) and of shelf life. The overall content in SO2 could also be reduced as sugar is a preservation agent, the product will be close to a fruit "paste".
8.4.5 Reconstitution test for dried/dehydrated products
In reconstitution water is added to the product which is restored to a condition similar to that when it was fresh. This enables the food product to be cooked as if the person was using fresh fruit or vegetable.
All vegetables are cooked but many of the dried fruits can be used for eating after they have been soaked in water. The following reconstitution test is used to find out the quality of the dried product.
Reconstitution test
This test can be used also to examine dried products after they have been stored for some time. Evaluation of rehydration ratio may be performed according to the following calculations.
Rehydration ratio. If weight of the dried sample is 10 g (Wd) and the weight of the sample after rehydration is 60 g (Wr), rehydration ratio is:
Rehydration coefficient. The weight of rehydrated sample is 60 g (Wr); the weight of dried sample is 10 g (Wd) and its moisture is 5% (Wu); raw material before drying had 87% water (A); rehydration coefficient is:
A simpler test for eating quality can be carried out without weighing and measuring. The material is placed in a cooking pot with water (and a little salt). The pot is then covered and boiled as described above.
Except for a few products which are eaten in the dry state, most dried fruit and all dried vegetables are prepared by soaking and cooking. Often this preparation is carried out incorrectly and dried products get a bad reputation.
Good quality dried products, after cooking and if properly treated should be similar to cooked fresh produce. In order to get good results, the following methods are recommended:
Quick method. - Cold water, ten times the weight of the dry product, is added to the dried product. The container is covered, brought to the boil and simmered GENTLY until the product is tender. The cooking time may be 15 to 45 minutes after the boiling point has been reached.
Slow method. - This gives better results than the quick method. Cold water is added to the dry food and is left to soak for 1 to 2 hours before cooking. The product is then cooked in the same water as that in which it was soaked. The actual cooking time will probably be shorter than that for the quick method.
Other points to remember are:
The composition of some dried fruits is seen in Table 8.5.2.
TABLE 8.5.2 Composition of some dried fruits
| Fruit | Moisture content % | Sugar as monosaccharides, % | Other carbohydrates | Vitamin C mg/100 g |
| Raisins | 21.5 | 64 | 7 | 0.0 |
| Dates | 12.5 | 55 | 7 | 0.0 |
| Prunes | 20.0 | 45 | 15 | 0.0 |
| Apricots | 15.0 | 45 | 24 | 4.8 |
| Peaches | 15.5 | 55 | 14 | 8.0 |
8.4.6 Handling, sorting, packing and storage of dried and dehydrated fruit and vegetables.
It is not easy to assess when drying has been completed. In absence of instrumentation, the characteristics of the various products after drying / dehydration can only be assessed by experience. Although this cannot be conveyed adequately on paper, some general indications can be given.
Fruit products. - When a handful of fruit is squeezed tightly together in the hand and then released, the individual pieces should drop apart readily and no moisture be left behind on the hand. It should not be possible to separate the skin by rubbing unpeeled fruit and the fruit centre should no longer reveal any moist area. Banana should be leathery and not too tough to eat in their dry state.
Vegetable products. - Onions should be dried until they are crisp whereas tomatoes should be leathery.
In general, the lower the moisture content, the better the keeping quality will be, but overdried products generally have an inferior quality. Also the loss in weight from excessive drying cannot be tolerated in a commercial operation designed to run profitably.
It is, however, essential to dry up to an optimum / safe moisture level, related to the type of the product and his designed shelf life, and to avoid running the risk of the products becoming spoiled due to excess water content.
When drying is completed, the material should be sorted either on trays or on a table in order to remove pieces of poor quality and colour and any foreign matter.
Very fine material should be separated from the bulk of the material by using a sieve (12 or 16 mesh per inch). Bad quality products which show poor colour need to be removed from the bulk of finished product.
After selection and grading, dried products should be packed immediately, preferably in polythene bags which must be folded and closed / tied tightly. However, plastic bags are easily damaged and therefore they must be packed into cartons or jute sacks before they are transported.
8.4.7 Deterioration of dried fruit during storage
Dried fruit must be considered as a relatively perishable commodity in the same category as cereals, pulses and similar stored products. It is subject to deterioration resulting from mould growth, insect and mite infestation and physical and chemical changes.
8.4.7.1 Mould growth
When the moisture content of dried fruit is allowed to exceed the maximum permissible level for safe storage then mould growth may occur. Table 8.4.1 indicates the moisture levels applicable to various types of fruit; and it can be seen that the safe moisture levels for dried fruit are much higher than those for other similar commodities.
At the present time, suitable field moisture meters for use with dried fruit are not readily available, and moisture determinations can only be satisfactorily carried out where laboratory facilities are available.
Various species of drought resisting fungi may develop on dried fruit when the moisture content is just above the safe level, and a number of osmophilic yeasts are quite commonly associated with spoilage in dried fruit.
Many of the yeasts bring about fermentation with the production of lactic acid or alcohol, and yeasts are frequently present in wart-like crystalline growths which occur in fruit which has become "sugared". In very moist fruit mucoraceous fungi may predominate and are visible as white fluffy growths on and within the fruit.
8.4.7.2 Mite infestation
Severe mite infestations are often associated with the growth of osmophilic yeasts in fermenting dried fruit products. Many of these mites are unable to complete their development in the absence of yeast. They have been reported as occurring on dried fruit, and particularly figs and prunes in Mediterranean countries. Such infestations are difficult to eradicate and affect consumer acceptance of the contaminated products.
8.4.7.3 Insect infestation
Insect infestations may begin in the field before harvest, may continue during bulk storage after drying, and unless measures are taken to prevent it, may occur in the finished packaged product during storage prior to distribution and consumption.
Regular treatments of the stack of dried fruit with a suitable insecticide will be necessary as a routine to combat light insect infestations. Pyrethrins synergised with piperonyl butoxide are commonly used as a surface spray or as an aerosol fog for this purpose. Heavy infestations will require that the fruit be fumigated.
8.4.8 Equipment for "dry sulphuring of fruit before dehydration/drying
SO2 generator
(1) SO2 outlet pipe
(2) Sulphur feeding door
(3) Plate for sulphur burning
(4) Small door
(5) Burner for gas
(6) Metallic sieve
Sulphuring installation
(1) Exhaust pipe
(2) SO2 pipe
(3) SO2 generator
(4) SO2 distributor
(5) SO2 distribution pipes
(6) Sulphuring cells
(7) SO2 flow regulator
(8) Pipe for removing SO2 from sulphuring cells
(9) Electrical fan
(10) Exhaust pipe for removing SO2 from sulphuring installation
(11) Hole for SO2 coming from cells
8.4.9 Solar dryer with air heater - tunnel type; community or business level
I. Air heaters
1.1 Cover: Transparent 3 mm. plexiglass plate or plastic sheet
1.2 Absorber - bottom - corrugated black painted iron sheets - intermediary - black
painted iron sieve
1.3 Insulation: none
1.4 Frames: metallic black painted corner/angle ironR
II. Drying tunnel or room
2.1 Cover: transparent plastic sheets
2.2 Frames: metallic black painted angle iron
2.3 Insulation: for back only on the outside
2.4 Back wall: wood plate, painted black on inside
2.5 Front wall: transparent plastic sheets
2.6 Side doors: - frames = wood
- transparent plastic sheets
2.7 Dimensions of drying tunnel / room
- L = 22 feet
- W= 18 feet
- H = 5 feet
2.8 One range of trolleys
III. Trays
3.1 Frames: wood, black painted
3.2 Bottom: nylon mesh or wood slats
3.3 Dimensions:
- l = 3 feet
- w = 2.4 feet
IV. Trolleys
4.1 Metal platform with wheels
4.2 Number of trays per trolley: 15
4.3 Distance between trays: 3"
4.4 Total height of trolley: 3 x 15 = 45 inch + 4 " (wheel) = 49 inch
V. Capacity calculation; number of trolleys and trays
5.1 Hypothesis: 300 kg fresh apricots received per day
2 days solar drying
5.2 Tunnel / dryer capacity: 2 x 300 = 600 kg fresh apricots
5.3 Number of trolleys and trays calculation:
- number of trays /trolley = 15
- kg fresh apricots per tray = 6 kg
- kg fresh apricots per trolley: 6 x 15 = 90 kg
- number of trolleys needed in drying room: 600 : 90 = 7
- number of trolleys in sulphuring cells = 2
- number of trolleys in "wet preparation section" = 3
- number of trolleys in dry section (after tunnel) = 2
- total number of trolleys needed = 14
- total number of trays needed: 14 x 15 = 200
VI. Sulphuring cell
6.1 Capacity: 2 trolleys
6.2 Dimensions: 1 = 5 feet; w = 3.5 feet; H = 55 inch
6.3 One sulphuring pipe inside the cell, diameter = 15 cm
VII. SO2 generator: diameter = 45 cm; h = 70 cm
VIII. Air heater / heat collector
8.1 DIMENSIONS: L = 5 M; H = 30 CM; Hl = 15 CM; H2 = 20 CM
8.2 Bottom / corrugated black painted iron sheets
8.3 Intermediate heat absorber: iron black painted metallic plate / sieve with holes of
about 3 cm in diameter fixation: at mid-distance between cover and bottom of the
heat collector
8.4 Cover: transparent 3 mm Plexiglas plate (or transparent plastic sheets)
The calculation used in this dryer is based on a real situation in a developing country, within the framework of FAO projects; availability of building materials was confirmed.
For the orientation of the dryer geographical position of the available drying yard and local wind direction were taken into consideration